Mizuguchi has confirmed to Kotaku that he is indeed working on an all new intellectual property; we just don't know what.
In case you're unaware of the man's credentials then lookup Space Channel 5 and Lumines with its numerous spin-offs. Rez recently came to XBLA; its first home was Dreamcast.
Rumblings suggest a 360 title.
Rez is a trance music shooter and its migrated from SEGA's immortal Dreamcast, to Sony's steadfast PS2 and finally to Microsoft's XBLA theatre of dreams. Basically Mizuguchi likes rhythm to his games, literally, so it’s likely his new creation will follow the beat too.